1. Field of the Invention
The present invention relates to a switched reluctance motor and an initial activating method thereof, and more particularly to a switched reluctance motor and an initial activating method thereof in which an aligning pulse is applied to a stator in order to make a rotor wait at a normal torque generating region to be driven, thereby preventing the rotor from rotating in a direction reverse to a target rotating direction.
2. Description of the Related Art
FIG. 1 is a sectional view illustrating the structure of a general switched reluctance motor. FIG. 2 is a circuit diagram illustrating the circuit configuration of the general switched reluctance motor. The general switched reluctance motor will now be described in detail with reference to FIGS. 1 and 2.
As shown in FIGS. 1 and 2, the switched reluctance motor (SRM) includes a driving control unit (not shown), a stator 20 carrying field coils W1 to W3 wound thereon while receiving current from the driving control unit, and a rotor 10 arranged inside the stator 20, and adapted to rotate in one direction by a reluctance torque generated between the stator 20 and the rotor 10 when current flows through the field coils W1 to W3.
The stator 20 includes a yoke having a cylindrical structure opened at upper and lower ends thereof, and a plurality of poles radially protruded from the inner surface of the yoke toward the rotor 10 while being uniformly spaced apart from one another in a circumferential direction. Field coils W1 to W3, which are also included in the stator 20, are each wound on a respective protruded pole 21. The number of protruded poles 21 may be determined in accordance with the kind of the motor used, even though the illustrated stator 20 has six poles.
The rotor 10 includes a rotor core 18 having a laminated structure. The rotor core 18 is provided with six poles 11 protruded from the outer surface of the rotor core 18 while being uniformly spaced apart from one another in a circumferential direction. A rotating shaft 17 is axially mounted to the central portion of the rotor 10 so that it rotates along with the rotor 10, and externally transmits the driving force of the motor. A pair of bearings 19 are arranged at upper and lower portions of the rotating shaft 17 in order to rotatably support the rotor 10. The rotor core 18 is arranged between the upper and lower bearings 19.
The driving control unit receives a sensing signal from a sensor 30, such as a photo sensor or Hall sensor, adapted to sense the position and speed of the rotor 10, generating driving pulses for switching on/off switches SW1 and SW2 connected to respective field coils of each field coil pair including two field coils W1, W2 or W3 facing each other in order to allow current to flow through the field coil pairs respectively associated with the field coils W1, W2, and W3 in a sequential fashion.
The switching-on/off operations of two switches SW1 and SW2 are simultaneously carried out. In accordance with the simultaneous switching-on operations of the switches SW1 and SW2, the facing field coils W1, W2, and W3 are electrically connected, so that current from the driving control unit flows through those field coils. As the current flows through the field coils W1 to W3, a reluctance torque is generated between the stator 20 and the rotor 10, causing the rotor 10 to rotate in a direction of a minimum magnetic reluctance.
The above-mentioned switched reluctance motor is configured to make the rotor 10 wait at a particular position to be driven, using a magnetic force, in order to allow the rotor 10 to rotate in one direction. That is, the switched reluctance motor is provided with a ring magnet 15 arranged around the rotor 10 above the rotor 10 having a ring shape, and a parking magnet 16 arranged to face the ring magnet 15, and interacting with the ring magnet 15 to generate an interactive magnetic force. When the rotor 10 is stopped, an attractive magnetic force is generated between the ring magnet 15 and the parking magnet 16, causing the rotor 10 to be maintained at a particular position at which the rotor 10 can generate a torque for rotation in a normal direction.
The number of poles formed in the ring magnet 15 is determined based on the number of protruded poles provided at the motor. That is, when the number of protruded poles is n, the ring magnet 15 includes n N-poles, and n S-poles. On the other hand, the parking magnet 16 has one N-pole, and one S-pole, irrespective of the number of protruded poles in the motor.
When the switched reluctance motor is stopped, its rotor is positioned at a normal torque generating region or a reverse torque generating region. Such states of the rotor are shown in FIGS. 3 and 4, respectively. The switched reluctance motor shown in FIGS. 3 and 4 has a configuration including a rotor 10 having 6 protruded poles 11, and a stator 20 having 6 protruded poles 21. It is assumed that the target rotating direction, that is, the normal rotating direction, corresponds to a counterclockwise direction.
When current is applied to the protruded pole A of the stator 20 in a state in which the protruded pole A′ of the rotor 10 does not move, it causes the protruded pole A′ of the rotor 10 to rotate in a direction that causes the protruded pole A′ to be aligned with the protruded pole A of the stator 20. That is, the protruded pole A′ of the rotor 10 rotates in the normal, or counter-clockwise direction. The region where torque-causing rotation in the normal direction is generated is referred to as a “normal torque generating region.” Reliable control is achieved in so far as the application of the driving current is carried out under the condition in which the protruded pole A′ of the rotor is maintained at the normal torque-generating region.
The positional relation between the ring magnet 15 and the parking magnet 16 under the above-described condition in FIG. 3 will be described. One N-pole of the ring magnet 15 faces the S-pole of the parking magnet 16 such that its pole boundary line is aligned with the pole boundary line of the parking magnet 16. In this state, a maximum attractive force is generated between the ring magnet 15 and the parking magnet 16. By this maximum attractive force, the magnet torque generated between the ring magnet 15 and the parking magnet 16 becomes zero. As a result, the ring magnet 15 no longer rotates, so that it is stably maintained at a position where the rotation is stopped. The position where the rotation of the ring magnet 15 is stopped, that is, where the ring magnet 15 and parking magnet 16 are in stable equilibrium, in accordance with the magnet torque rendered to be zero, is referred to as a “stable equilibrium position (SEP).”
However, when the ring magnet 15 and parking magnet 16 have a positional relation shown in FIG. 4, the rotor 10 cannot be maintained at the normal torque-generating region. In other words, when one S-pole of the ring magnet 15 faces the S-pole of the parking magnet 16 such that its pole boundary line is aligned with the pole boundary line of the parking magnet 16, a maximum repulsive force is generated between the ring magnet 15 and the parking magnet 16. By this maximum repulsive force, the magnet torque generated between the ring magnet 15 and the parking magnet 16 becomes zero. As a result, the ring magnet 15 does not rotate, so that the ring magnet 15 and parking magnet 16 are in equilibrium.
In this state, however, the alignment between the pole boundary lines of the magnets 15 and 16 may be easily lost even when a small rotating force is applied to the ring magnet 15, because the equilibrium between those magnets are not maintained by the attractive force serving to attract the magnets toward each other, but maintained by the repulsive force serving to repulse the magnets away from each other. The moment the poles of different polarities between the ring magnet 15 and the parking magnet 16 face each other due to the loss of the pole boundary line alignment, a substantial torque is generated between the magnets 15 and 16. By this torque, the ring magnet 15 may be rotated in an unspecified direction. The position where the ring magnet 15 and parking magnet 16 are maintained in an equilibrium state only for a moment, that is, an unstable state, is referred to an “unstable equilibrium position (UEP).”
When the magnets 15 and 16 have a positional relation causing the unstable equilibrium, the pole A′ of the rotor 10 is finally positioned within an angular range of −30° to 0°. When current is applied to the protruded pole A of the stator 20 in this state, it causes the protruded pole A′ of the rotor 10 to rotate in a direction causing the protruded pole A′ to be aligned with the protruded pole A of the stator 20, in a clockwise direction. That is, a torque causing rotation in a reverse direction is generated. The region where such a reverse torque is generated is referred to as a “reverse torque generating region.” When driving current is applied in a state in which the protruded pole A′ of the rotor 10 is maintained at the reverse torque generating region, the rotor 10 is rotated in a direction reverse to the target rotating direction. As a result, an appliance equipped with the motor may be abnormally controlled. Moreover, the durability of the motor is reduced. In severe cases, the appliance may break down.